The following filters have been conventionally used as a heat ray-absorbing filter or as a filter for adjusting the visibility of video camera:                (1) a filter composed of phosphate glass containing metallic ions such as copper or iron ions (Japanese Unexamined Patent Publication No.235740/1985, Japanese Unexamined Patent Publication No.153144/1987, etc.);        (2) an interference filter having plural layers differing from each other in refractive index on a substrate to allow light of specific wavelength to pass by interference of transmitted light (Japanese Unexamined Patent Publication No.21091/1980, Japanese Unexamined Patent Publication No.184745/1984, etc.);        (3) an acrylic resin filter composed of a copolymer containing copper ions (Japanese Unexamined Patent Publication No.324213/1994); and        (4) a filter composed of a binder resin and a coloring matter dispersed in the binder resin (Japanese Unexamined Patent Publication No.21458/1982, Japanese Unexamined Patent Publication No.198413/1982, Japanese Unexamined Patent Publication No.43605/1985, etc.).        
The above-mentioned conventional infrared absorption filters have problems as described below.
The filter (1) exhibits sharp absorption in the near-infrared region and can intercept infrared radiation at a very high ratio. However, the filter (1) pronouncedly absorbs part of red color in the visible light region so that the transmitted color looks blue. For display purposes, importance is laid on a color balance. In such case, it is difficult to use the filter (1). Another problem is raised about the processability of the filter (1) because it is made of glass.
The optical properties of the filter (2) can be freely designed. Further a filter having properties almost equal to the designed properties can be produced. However, the filter (2) necessitates a plurality of layers differing in refractive index from each other for this purpose, consequently entailing a drawback of incurring high production costs. Moreover, when a large area is required, the filter (2) should have a uniform thickness of high precision over the entire area, resulting in a difficulty in producing the filter.
The filter (3) has improved processability compared with the filter (1). However, the filter (3) exhibits sharp absorption and absorbs the red color of light beams as is the case with the filter (1), raising the same problem as the filter (1) that the filter (3) looks blue.
In the filter (4), various infrared-absorbing materials can be used. Examples of useful materials are phthalocyanine, nickel complex, azo compound polymethine, diphenylmethane, triphenylmethane, quinone and the like. However, when singly used, these materials pose problems of showing insufficient absorption or absorbing a visible light of specific wavelength in the visible light region. Further, when the filter is left to stand at a high temperature or a high humidity for a long time, the infrared-absorbing materials involve problems of decomposing or oxidizing, bringing about absorption in the visible light region or ceasing absorption in the infrared region.
An object of the present invention is to provide an infrared absorption filter which can achieve absorption in the near-infrared region, the filter showing a high transmittance in the visible light region, being free from marked absorption of a light of specific wavelength in the visible light region, and being excellent in environmental stability and in processability and productivity.